This invention relates to integrated circuits (ICs), and more particularly to disabling substrate bias generators.
Charge pump circuits are commonly used for generating voltages above or below supply voltages. Memory circuits often use a charge pump or substrate bias generator to generate a substrate bias Vbb that drives the p-substrate or P-wells under n-channel transistors. Using a substrate bias below ground increases the width of the depletion region under a transistor and thus reduces parasitic capacitances. Speed is thus improved.
More recently substrate bias generators have also been used to improve undershoot protection for bus-switch transistors. See Kwong, U.S. Pat. No. 6,052,019, assigned to Pericom Semiconductor Corp. of San Jose, CA.
The oscillator that pumps the charge in a substrate bias generator draws power. A voltage comparison can be performed to compare the substrate voltage to a target, and the charge pump turned off when the target is reached, saving power. See Hsiao et al. in U.S. Pat. No. 5,694,072, assigned to Pericom Semiconductor Corp. of San Jose, Calif.
To prevent pumping below the target Vbb, a clamp has been used to quickly discharge the substrate until the target is reached. See Vajdic, U.S. Pat. No. 4,794,278, assigned to Intel Corp. of Santa Clara, CA. Other devices vary the frequency of the oscillator and also detect when the power supply is too high. See Yoo et al., U.S. Pat. No. 5,889,719, assigned to Samsung Electronics Co. of Korea.
Detection of the Vcc over-voltage condition is useful, since the substrate generator pumps to a lower target when Vcc is increased. If the substrate voltage reaches the breakdown voltage of the transistors or junctions, damage to the IC may result. Vcc supply over-voltage may occur during electro-static-discharges (ESD) events, resulting in chip damage when no over-voltage protection is provided for the charge pump.
A low-power substrate-bias generator that includes Vcc supply over-voltage protection is desirable.